1. Field of the Invention
The present invention relates to radar sensors, and more particularly to short-range pulse-radar sensors having a constant false alarm rate (CFAR) detector.
2. Description of Related Art
CFAR detection is a digital detection process for radar where the detection threshold is increased in the presence of noise or radar clutter. In large radars, such as air traffic control radars, clutter usually dominates rather than internally generated noise. These radars often use range cell averaging to set the digital threshold. Noise due to rain, clutter returns or interference automatically raises the detection threshold across each range cell and prevents the display from becoming obliterated. A discussion of cell-averaging CFAR detectors can be found in radar texts.
Small radar sensors such as 10.5 GHz CW Doppler radars commonly used to open doors or sense intrusion into a secure area are prone to false alarms due to RF interference from nearby radar sensors or from the increasing number of communications devices radiating in the microwave spectrum, particularly in the ISM bands at 900 MHz and 2.4 GHz. While these Doppler radars have front-end RF filters to reject out-of-band interference, there is little that has been done to prevent false alarms from in-band interference.
Ultra-wideband Micropower Impulse Radar (MIR) as exemplified in U.S. Pat. No. 5,361,070, xe2x80x9cUltra-Wideband Radar Motion Sensor,xe2x80x9d to McEwan, and wideband motion sensors based on pulsed RF emissions such as U.S. Pat. No. 5,966,090, xe2x80x9cDifferential Pulse Radar Motion Sensor,xe2x80x9d to McEwan rely on a high level of pulse averaging to reject in-band interference. Typically, 1000 pulses are averaged, providing 30 dB of in-band interference rejection, which may be an insufficient amount of rejection if an in-band interferer is within several meters range. Some applications require zero false alarms due to RF interference at any range.
The proliferation of cell phones and wireless LANs, and an unpredictable future in terms of RF interference leave many prospective users of radar sensors wary of false alarm nuisances. For example, a 5.8 GHz radar motion sensor may rarely trigger from RF interference today, but perhaps two years from now 5.8 GHz phones, wireless LANS, or some unknown product may proliferate and turn a great radar sensor into a major false alarm headache.
Clearly, xe2x80x9cno false alarmsxe2x80x9d must be ensured if radar sensors are to find their way into widespread use. A radar sensor having a CFAR detector of the present invention overcomes these prior limitations.
The present invention is based on range-gated pulsed radar having randomized pulse-to-pulse emissions in the form of a randomized pulse repetition frequency (PRF). For each pulse emitted, the radar generates a range gate pulse to gate its receiver at a precise time after emission, corresponding to a precise target range. The range gate timing relative to an echo return is not affected by the randomized PRF, since neither the range gate timing nor the round trip echo time vary with PRF.
However, a random PRF randomizes the pulse-to-pulse gate time such that RF interference is gated, i.e., sampled, randomly. All RF interference is thereby converted into a sequence of random amplitude samples. This random sequence of samples has a broader spectral width that the desired radar signal, thereby allowing filters to separate signal and interference into two channels (or processors), a signal channel (or processor) and an interference channel (or processor). The interference channel is rectified and low-pass filtered (i.e., envelope detected) to provide a reference level for a CFAR threshold detector. By suitably adjusting the gain of the interference channel, increasing levels of RF interference can be made to increase the CFAR threshold faster than the increase in interference in the signal channel signal, thereby assuring no CFAR threshold crossings due to RF interference. In effect, the radar becomes increasingly blinded with RF interference but never false triggers due to RF interference. Thus, the CFAR detector only produces an output when the radar transceiver has received an echo signal from the target and not from an interference signal.
Unlike previous CFAR detectors which average noise from multiple range cells to set a CFAR threshold in each range cell (i.e., channelizing in time without randomizing the PRF), the present invention randomizes all RF interference by randomizing its PRF and accordingly its sampling rate, and then spectrally filters the sampled receive RF into a signal channel and an interference channel for CFAR detection purposes (i.e., channelizing in frequency with a randomized PRF). A primary object of the present invention is to provide a radar sensor free of false alarm nuisances due to RF interference.
Another object of the present invention is to provide a spread-spectrum microwave motion sensor that can be collocated with other spectrum users without having to set a specific operating frequency.
Uses for the present invention include indoor and outdoor security alarms, home automation and lighting control, industrial and robotic controls, automatic toilet and faucet control, tank level sensing, rangefinding, automatic door openers, vehicle backup warning and collision detection, and general appliance control.